1. Field of the Invention
The present invention relates to a lithium secondary battery comprising a cathode made by sintering a hthium-transition metal oxide and having a high battery capacity and excellent charge/discharge cycle characteristic, and a method of producing the same.
2. Description of Related Art
With the popularization of cellular phones and notebook computer, the lithium secondary batteries that are capable to provide a high-energy battery density have attracted much attention. The lithium secondary battery comprises a cathode and an anode both including an active material capable of incorporating and releasing lithium ions, and a lithium ion conductive electrolytic solution or solid electrolyte. However, there is such a problem that the electrode includes such materials as a binder and an electrically conductive material that do not contribute to the battery capacity, thus resulting in a limitation to the capacity per volume of the battery.
As means for increasing the capacity per volume of the battery, an attempt to make the electrode from a sintered material, which is substantially made of an active material. When the electrode is constituted from a sintered material made of an active material, no binder is included and the addition of an electrically conductive material can be elimated or reduced, thus making it possible to increase the active material filling density and increase the capacity per volume. For example, Japanese Laid-Open Patent Publication No.8-180904 discloses a cathode made of a sintered lithium-transition metal oxide. According to this disclosure, powder of a lithium-transition metal oxide or raw material powder thereof is pressed to form a molded material by using a die, with the mold material being fired at a predetermined temperature in the presence of oxygen, thereby to obtain a sintered material. However, an electrical conductivity of the sintered material is not sufficient for the cathode, and therefore it is necessary to improve the performance further.
For making the lithium secondary battery thinner, it is effective to reduce the thickness of the cathode and the anode that make up most of the thickness of the battery. In order to reduce the thickness of the cathode made of a sintered material, it is necessary to increase the surface area of the sintered material for securing a predetermined battery capacity. When the sintered material is made by the press molding, however, increasing the area of the die for the purpose of increasing the surface area of the sintered material makes it difficult to fill the cavity of the die uniformly with the powder of lithium-transition metal oxide or raw material powder thereof, resulting in unevenness of the thickness and a density of the molded material in a planer direction. As a result, sintering reaction does not proceed uniformly in the molded material, thus resulting in unevenness in the density of the sintered material in the planer direction. When such a sintered material is used as the electrode in a battery, there have been problems of a decrease in the battery capacity and poor charge/discharge cycle characteristic. In case there is a portion where sintering reaction has not progressed enough, on the other hand, bonding strength between primary particles that constitute the sintered material decreases in the portion, resulting in lower mechanical strength of the sintered material This leads to such problems, as the electrode is likely to disintegrate during charging or discharging, and a decrease in the battery capacity and poor charge/discharge cycle characteristic.
In case a current collector is pressed to the sintered material of lithium-transition metal oxide to form a laminate, there is a significant contact resistance between the current collector and the sintered material, leading to a filure in improving the battery capacity and the charge/discharge cycle characteristic. To counter this problem, for example, Japanese Laid-Open Patent Publication No.8-180904 described above discloses a method of decreasing the contact resistance by sintering the powder of a lithium-transition metal oxide or raw material powder of a lithium-transition metal oxide and, at the same time, integrating the sintered material with a current collector of aluminum. However, since sintering and integration with the current collector are carried out simultaneously, the firing temperature cannot be made sufficiently high. As a result, the sintering process tends to be insufficient thus leading to lower strength and/or lower electrical conductivity of the sintered material, resulting in insufficient improvement in the charge/discharge cycle characteristic.
Also when producing a battery wherein at least the cathode is made of a sintered material, the electrode cannot be wound as in the case of the conventional coated electrode because the sintered material has a low bending strength. When an electrode unit consisting of one sintered cathode and one sintered anode is to be assembled, for example, both electrodes can be easily aligned with each other simply by stacking the cathode and the anode to oppose each other while interposing a separator therebetween. However, when a battery having an electrode unit consisting of a number of pairs of cathode and anode is to be assembled for the purpose of achieving a large battery capacity, a plurality of cathodes and anodes must be accurately aligned to oppose each other via separators. This leads to a longer period of time for stacking the electrodes and the electrode unit, or requires it to use a high precision apparatus for alignment. Also there has been such a problem that, when moving a stacking electrode unit or housing the stacking electrode unit in a battery casing after the stacking process, the electrodes are shifted from the predetermined positions, thus leading to a decrease in the area over which the mating electrodes face each other, and resulting in a decrease in the battery capacity of the completed battery. Moreover, there has been such a problem that a current collecting lead wire is required for each electrode to ensure conduction to the plurality of cathodes and the anodes, thus giving rise to the difficulty of disposing the lead wires.
An object of the present invention is to provide a lithium secondary battery that, by providing a cathode of larger surface area and higher mechanical strength, has a large battery capacity and excellent charge/discharge cycle characteristic.
Another object of the present invention is to provide a lithium secondary battery that, with a current collector being integrated with a sintered material of a lithium-transition metal oxide without lowering the mechanical strength and electrical conductivity thereof, has a large battery capacity and excellent charge/discharge cycle characteristic.
Still another object of the present invention is to provide a lithium battery that comprises the electrode made of a plurality of sintered materials, where the cathodes and the anodes will not be shifted from the predetermined positions and high reliability is ensured.
The present inventors completed the present invention by finding out that the electrical conductivity can be used as an index of the bonding strength between primary particles that constitute a sintered material when forming the sintered material of a lithium-transition metal oxide, and that sufficient mechanical strength can be obtained by using a sintered material having a high electrical conductivity.
The lithium secondary battery of the present invention includes a cathode and an anode, each electrode containing an active material capable of storing and releasing lithium ions, wherein the cathode is a porous sintered material made of a lithium-transition metal oxide that has a porosity in a range from 15 to 60% and an electrical conductivity of more than 0.1 mS/cm. According to the present invention, since the porosity of the sintered material that constitutes the cathode is in a range from 15 to 60%, an electrolytic solution infiltrates sufficiently into the sintered material under such a condition that filling density of the active material is maintained at a high level. With this constitution, the internal electrical resistance can be decreased without decreasing the battery capacity. Also by sintering enough to achieve electrical conductivity of more than 0.1 mS/cm, high bonding strength between primary particles of the sintered material can be achieved so that the primary particles do not come off and the electrode does not disintegrate even when the sintered material expands and shrinks during charging and discharging cycles of the battery. High mechanical strength also makes it possible to form the cathode of larger surface area. Thus the cathode having the porosity in a range from 15 to 60% and the electrical conductivity of more than 0.1 mS/cm provides the battery with a large battery capacity and excellent charge/discharge cycle characteristic.
A method of producing a lithium secondary battery including a cathode and an anode, each containing an active material capable of incorporating and releasing lithium ions according to the present invention, cathode being made by sintering the lithium-transition metal oxide at a temperature higher than the melting point of the current collector, the method includes the steps of pressing the sintered material to the current collector, and heating at a temperature lower than the melting point of the current collector so as to join the sintered material to the current collector, thereby integrating the sintered material and the current collector. Since sintering is carried out at a temperature higher than the melting point of the current collector, sintering reaction can be accelerated and therefore the strength and electrical conductivity are increased. Further, since the sintered material and the current collector are integrated at a temperature lower than the melting point of the current collector, so that the current collector is not damaged by thermal deformation, and the contact resistance can be decreased. Consequently, strength and electrical conductivity of the sintered material are improved, and the cathode having lower contact resistance improves the charge/discharge cycle characteristic.
In the method of producing the lithium secondary battery according to the present invention, the step of forming the sintered material includes a) adding a binder and a solvent to a cathode material consisting of the powder of a lithium-transition metal oxide, thereby to prepare a coating solution; b) applying the coating solution to a base material and removing the solvent to form the coating film; and c) firing the coating film in the presence of oxygen to sinter the cathode material, thereby to form the sintered material. Since the coating film containing the cathode material consisting of the powder of a lithium-transition metal oxide is fired to form the sintered material, the cathode having a larger surface area and an uniformity in thickness and density can be obtained. The cathode can improve the battery capacity and cycle characteristic.
The method of producing the lithium secondary battery according to the present invention is capable of forming the sintered material with a uniform thickness and pressing the sintered material to the current collector. The sintered material with uniform thickness makes it possible to press the current collector uniformly over the entire surface of the sintered material. As a result, adhesion between the sintered material and the current collector can be improved and the electrical contact resistance can be decreased without causing the sintered material to deform and crack when pressing.
The lithium battery of the present invention is a battery comprising a stacked electrode formed of a multilayered electrode unit which includes cathodes and anodes piled via a separator and has strip-shaped current collector, wherein at least the cathodes are sintered materials which are aligned on and joined to one of the current collectors and spaced from one another at bending portions defined by desirable intervals on the current collector, and the cathodes and the anodes are disposed in the stacked electrode, with each anode opposed to the respective cathode. The cathodes and the anodes are accurately aligned so that the electrodes will not be shifted from the predetermined positions, and a current collection can be easily made since providing only one lead wire for each of the cathodes and the anodes suffices, and therefore a lithium secondary battery of high reiability can be achieved.
The battery including the stacked electrode described above is produced by a method, which includes the step of forming at least a cathode electrode sheet, on at least one side of the strip-shaped current collector, having a plurality of sintered material electrodes aligned on and joined thereto and spaced from one another at bending portions defined by desirable intervals on the current collector; the step of bending the stack which includes the cathode electrode sheet and an anode electrode sheet piled via a separator so that each cathode oppose the respective anode; and the step of housing the stack in a battery casing.